1. Field of the Invention
This invention relates to improved ceramic compositions and a method of toughening zirconia ceramics. More specifically, but not by way of limitation, this invention relates to the alloying effect on transformability of yttrium stabilized tetragonal zirconia polycrystals, Y-TZP.
2. Description of the Prior Art
It is generally known that zirconium dioxide (zirconia) exists in three allotropic forms; monoclinic, tetragonal and cubic and that there is a large volume expansion during the transition from tetragonal to monoclinic. Further, it has been historically accepted that because of this disruptive phase transition, the refractory properties of pure zirconia cannot be used. However, recent developments relating to suppressing or disrupting the deleterious effects of the phase transition have been discovered. For example, in the so-called partially stabilized zirconia (PSZ), the addition of metal oxide is viewed as creating a multiphase material having a fine-scale precipitate of tetragonal zirconia in a stabilized cubic matrix which in turn results in enhanced toughness and strength. More recently, an even more powerful strengthening mechanism has been suggested involving the toughening and retaining of the metastable tetragonal zirconia to room temperature. In this development, the martensitic transformation (fast and diffusionless) between monoclinic and tetragonal phases is partially alleviated by inducing and creating tetragonal zirconia in sintered bodies or domains of a resulting time-stabilized zirconia. Although these transformation stabilizing ceramic mechanisms and their underlying rationales may be questionable and although their respective effects on high temperature properties of the resulting transformation toughened ceramics are encouraging, the breadth of applicability of these general principles to the field of ceramics generally and the extent to which the properties can be improved is still not well defined or understood. Thus, the use of transformation toughened ceramics and ceramic coatings still remains uncertain.